1. Field of the Invention
The present invention relates to electronic display devices. More particularly, the present invention relates to an inverter circuit capable of driving a discharge tube, a backlight assembly including the inverter circuit, and a liquid crystal display (“LCD”) including the backlight assembly.
2. Description of the Related Art
Illustratively, discharge tubes may be implemented using cold cathode fluorescent lamps (“CCFLs”) as described hereinafter, but it is to be clearly understood that the present invention is not limited to CCFLs. For example, the present invention may be implemented in a system that turns on a plurality of discharge tubes in response to an applied alternating current (“AC”) voltage, wherein these discharge tubes are not construed as being limited to the CCFL.
A conventional LCD uses a CCFL as a backlight. In recent years, large LCD televisions have been developed which use correspondingly large LCD displays. Accordingly, plural CCFLs are used to provide a backlight for these large LCD displays.
FIG. 1 is a schematic view illustrating light emitting properties for a prior art CCFL 301. The CCFL 301 is a type of fluorescent lamp that operates in a normal glow discharge region. A phosphor 322 is coated inside a glass tube 321 of the CCFL 301, and a slight amount of inert gas and mercury are sealed within the glass tube 321. By applying an AC voltage between electrodes 328 disposed on both sides of the CCFL 301, a glow discharge occurs in mercury vapor. Due to this discharge, mercury 323 is excited and an ultraviolet ray 324 is generated. The phosphor 322 coated in the glass tube 321 is excited by the ultraviolet ray 324 to a high energy level. Light is emitted at a wavelength corresponding to an energy difference occurring when the excited phosphor atoms return to a low energy level from the high energy level. The CCFL 301 emits light having a wavelength determined by the phosphor atom. Also, the CCFL 301 has a negative resistance characteristic in that impedance is reduced as a function of increasing current flowing therethrough. Also, because it is difficult to fabricate the CCFLs having the same (or uniform) impedance, the impedances of the CCFLs are dispersed throughout an arbitrary range.
The following approaches have been proposed to solve problems occurring when the number of CCFLs increases. For example, a structure may be employed in which a number of inverter transformers increases according to the number of CCFLs used. As illustrated in the prior art configuration of FIG. 2, a plurality of inverter transformers 900A to 900N is provided to correspond to CCFLs 301 to 310, respectively. As the number of inverter transformers increases, the inverter transformers occupy an undesirably large area on a printed substrate. Therefore, a size of the inverter circuit becomes large.
To reduce the size of the inverter circuit, driving a plurality of CCFLs 301 to 310 using a single inverter transformer may be considered as illustrated in the prior art configuration of FIG. 3.
However, the structure of FIG. 3 causes interference with a driving circuit of the LCD because the CCFLs 301 to 310 are driven by a sinusoidal AC voltage 94A of a same polarity. Consequently, noise such as fringe interference is observed on the display screen. This noise can be eliminated or reduced by providing a differential type inverter transformer 901 as illustrated in the prior art configuration of FIG. 4. That is, the inverter transformer 901 is configured such that sinusoidal AC voltages 95 and 96 generated from two secondary coils have opposite polarities.
However, as described above, two secondary coils have to be constructed to provide opposite polarities with respect to each other in order to obtain voltages of reverse phase at the secondary sides of the inverter transformer 901 for a differential voltage implementation. It is difficult to obtain the AC voltages 95 and 96 for these reverse phases from the two secondary coils. When the AC voltages 95 and 96 of the reverse phases generated from the secondary coils of the inverter transformer 901 are not uniform, variations are observed in the currents flowing through the CCFLs 301 to 310, thereby causing bright areas or dim areas or both.
Also, as described above, the CCFLs have a negative resistance characteristic. When the CCFLs 301 to 310 are connected in parallel to the inverter transformer 901, it is assumed that a current begins to flow through a specific CCFL having a relatively low impedance compared with the remaining CCFLs of CCFLs 301 to 310. In this case, current is concentrated in the specific CCFL because the current flows more easily as the resistance of the specific CCFL decreases. As a result, the bright areas occur at one or more CCFLs, thereby shortening the lifespan of the CCFLs.
To avoid the aforementioned problem, a balance circuit may be connected in series with the CCFLs. FIG. 5 is a prior art circuit diagram illustrating an example of a balance circuit 400 connected to CCFLs 310 to 310. When a current flows through an arbitrary CCFL, a current flows through a primary coil of a balance transformer (for example, one of balance transformers 401 to 410 in FIG. 5) connected in series with the CCFL. This causes a current to flow through a secondary coil of the balance transformer. Since the secondary coil of the balance transformer is connected in series with the secondary coils of the remaining balance transformers, a current flowing through the secondary coils of the balance transformers forces a current to flow through the primary coils of the balance transformers 401 to 410. Consequently, currents of the respective CCFLs 301 to 310 are controlled in the same manner. As illustrated in FIG. 5, a loop formed by the secondary coils of the balance transformers 401 to 410 is grounded. A detected voltage is detected at a contact node (detection node) 501 in a state wherein a secondary coil of at least one balance transformer is interposed between a grounded node and the contact node (detection node) 501. The detected voltage is a voltage that is necessary for the balance transformers 401 to 410 to maintain balance of the CCFLs 301 to 310. The magnitude of the detected voltage is different according to the dispersion of the resistances including the negative resistance characteristic of the CCFLs. Using this voltage observation, an open circuit or a short circuit caused by malfunction of the CCFLs can be detected. That is, when the open circuit or the short circuit occurs, a higher voltage compared to a voltage at a normal state is generated at the detection node 501 so as to maintain the balance of the balance transformers 401 to 410.
[Related reference 1] Japanese Patent Laid-open Publication No. 2004-335443
[Related reference 2] Japanese Patent Laid-open Publication No. 2005-203347
When the impedance of a CCFL increases because the lifetime of the CCFL is nearly at an end, the Q of an inverter resonance circuit becomes high so that a relatively high voltage is generated. Therefore, a corona discharge is easily generated between a line disposed between the secondary coil of the inverter transformer and another line. The corona discharge gradually carbonizes an insulating coating of the lines, thereby causing short circuiting of the lines.
The balance transformer 400 used in the inverter circuit for turning on the CCFLs 301 to 310 for the backlight of the conventional LCD of FIG. 5 is connected to terminals of the CCFLs 301 to 310 which are opposite with respect to the inverter transformer 901. When an abnormal state such as a current concentration on a specific CCFL occurs, the balance transformer 400 generates a higher voltage relative to a normal state at the voltage detection node 501. Automatic operation of the control circuit is possible by detecting the voltage at the voltage detection contact point 501. However, when a high voltage discharge such as a corona discharge occurs between a line disposed between the secondary coil of the inverter transformer 901 and the CCFLs 301 to 310 and another line, this high voltage discharge does not influence the balance between the CCFLs 301 to 310. For this reason, it is virtually impossible to detect an abnormal state such as a high voltage discharge occurring in a voltage detection node of the balance transformers 401 to 410 connected to terminals of the CCFLs 301 to 310.